1. Field of the Invention
The present invention relates to the generation of an electrical signal commensurate with the level of a liquid and particularly to the determination of a fill state of interest for a liquid disposed in a container. More specifically, this invention is directed to a liquid level detector and especially to an actuator for a proximity type switch, the actuator being responsive to the level of a contained liquid. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
There are many situations where the level of a liquid within a container must be sensed in order to provide a warning or initiate an action when the level reaches a critical state, typically a "full" or "empty" state of the container. By way of example, it may be necessary to sense a "full" condition to initiate operation of a sump pump or it may be necessary to sense an "empty" condition in order to initiate replenishment of the fluid in a reservoir. Many liquid level responsive devices are known in the prior art. The known level responsive devices typically provide an output signal commensurate with one or more positions of a float which moves vertically as the level of liquid in a container rises and falls. For a very early version of a liquid level indicator which includes a float, reference may be had to Patent 531,106.
With the advent of reliable proximity switches, i.e., switches responsive to a magnetic field such as produced by a permanent magnet, liquid level indicating devices which utilized such switches were introduced. For examples of such level indicators which employ proximity or "reed" switches, reference may be had to Pat. Nos. 3,678,232 and 3,826,139. In the known liquid level detectors which employ proximity switches, a magnet will typically be incorporated into the float while the switch which cooperates therewith will be permanently secured adjacent the path of float travel at a level commensurate with the liquid level of interest. While very successful, the prior art liquid level sensors which employ proximity switches have been characterized by certain disabilities and disadvantages. For example, most of the prior art liquid level sensors which employ proximity switches employed a float which travels on a vertically oriented guide stem. Thus, both the float and the guide along which it travels are immersed in the liquid and are subject to contamination. The build-up of particulate matter on the float and/or stem has been a particularly prevalent problem which, when it occurs, causes the float to jam on the stem thus disabling the sensor. In those cases where an attempt has been made to mount the float of a level detector for movement other than in a vertical direction, the float has typically been hinged on a pin and both the float and pin have been in contact with the fluid. Thus, the non-vertical movement of the float has not solved the problem of contamination and subsequent jamming of the actuators of level detectors employing proximity switches.
Continuing to discuss the prior art, and as may be seen by reference to Pat. No. 3,826,139, the previous practice of mounting the magnet on the float results in both of the components of the sensor, i.e., the magnetic field generator and the field responsive signal generator, being exposed to conditions prevalent in the liquid. These conditions may include a very high or a very low temperature which can effect performance and, particularly, cause sufficient distortion of the contacts of the reed switch as to prevent operation. This potential problem is not overcome by reversal of the position of the field generator and signal generator.
In many operating environments a liquid level detector will be mounted from the wall of a container. In such installations the container wall is perforated to permit the mounting and/or to permit the completion of an electrical circuit to the switch of the level sensor. In many cases the liquid confined in the container will be at a significantly different temperature relative to the ambient environment about the container, the ambient environment consisting of a fluid which may be either gas or a liquid. This temperature difference may cause differential expansion or contraction between the sensor mounting and the container wall. Since the sensor will be immersed in the liquid within the container, any such differential expansion or contraction may result in leakage. Leakage may also result from deterioration of the seals about the perforation in the container wall if the liquid being contained reacts with the joint compound.
Yet another persistent problem with prior art liquid level sensors resides in the fact that they have historically performed poorly in highly viscous environments. This poor performance has resulted from the sticking of the floats on their associated stems or because the viscosity has otherwise prevented relative movement between the magnet and switch.